The present invention relates to a compound semiconductor field-effect transistor such as GaAs FET, InP FET or InGaAs FET, and more particularly to an atomic planar-doped field-effect transistor which has an atomic planar-doped layer in the channel region.
The compound semiconductor field-effect transistors are featured by their high carrier mobilities and high velocities in carrier saturation. They have been widely employed as high-frequency discrete transistors or in high-speed IC devices. However there is a demand to further increase their operation frequencies and operation speeds. For that purpose, it is required to make the device size smaller and the channel layer thinner, thereby increasing the transconductance while maintaining the gate-source breakdown voltage and the current driving capability. Schubert et al. have proposed the atomic planar-doped field-effect transistor to meet the requirements, the Schubert et al proposal being disclosed in IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-33, No. 5, May 1986, pp. 625-632. The proposed FET has a planar-doped layer within its channel region between the source and the drain that is an atomic monolayer where ionized donors extend two-dimensionally. This structure realizes a thinned channel while maintaining the gate breakdown voltage.
In the proposed FET, however, ion scattering causes a scattering of carrier electrons, which decreases the mobility at a low electric field. The decreased mobility at a low electric field causes an increase in the parasitic resistance of the device. Moreover, the overshoot effect is decreased due to an increase in ion scattering. It is impossible to expect a substantial improvement of the characteristics even if a device size is reduced.
Ploog et al. have presented a solution of the above problems, in which the atomic planar-doped layer is separated from the electron channel by the use of a heterojunction. The Ploog et al solution is described in the Journal of Crystal Growth, Vol. 81, 1987, p.304. However, the use of a heterojunction gives rise to another problem because the device characteristics become unstable due to a deep level which is generally present in heterojunction devices.